Attempt to parametrize runs-on

.github/workflows/spacktainer.yaml

@@ -48,10 +48,20 @@ jobs:
           DOCKERHUB_USER: ${{ secrets.DOCKERHUB_USER }}
           DOCKERHUB_PASSWORD: ${{ secrets.DOCKERHUB_PASSWORD }}
           SPACK_DEPLOYMENT_KEY_PUB: ${{ secrets.SPACK_DEPLOYMENT_KEY_PUB }}
+  get-builder-image:
+    runs-on: ubuntu-latest
+    env:
+      builder_image: LINUX_IMAGE-${{ secrets.AWS_ECR_URL }}/spacktainers/builder:latest
+    steps:
+      - name: Getting builder image
+        run: |
+          echo "We need at least one step, that's why this is here."
+    outputs:
+      builder: ${{ env.builder_image }}
   spacktainer-build-job:
     runs-on:
       - codebuild-spacktainers-${{ github.run_id }}-${{ github.run_attempt }}
-      - image:LINUX_IMAGE-130659266700.dkr.ecr.us-east-1.amazonaws.com/spacktainers/builder:latest
+      - image:${{ needs.get-builder-image.outputs.builder }}
       - instance-size:small
     needs: base-container-job
     steps: